This invention relates to fasteners, and particularly to a fastener assembly for securing a composite panel to a noncomposite support structure.
Gas turbine engines include several components exposed to hot combustion gases. Among these components are combustion chambers, exhaust nozzles and afterburner liners. Each of these components bounds a portion of a gaspath that channels the combustion gases through the engine. It is desirable to construct these components of heat tolerant materials. Ceramic matrix composites (CMC""s) are one class of materials known to possess the requisite temperature tolerance. CMC""s include materials comprised of carbon or silicon carbide fibers in a carbon or silicon carbide matrix. However the composite materials themselves may not be able to withstand the severe structural demands imposed by a gas turbine engine. Accordingly, the aforementioned components may be comprised of a noncomposite support structure and a heat tolerant composite panel secured thereto. The composite panel shields the metal support structure to maintain the support structure within its operational limits.
Securing composite panels to a support structure is a considerable technical challenge. Mechanical fasteners, such as nut and bolt assemblies, are attractive for securing a panel to its support because they allow for convenient removal of damaged panels. However metallic mechanical fasteners are unsatisfactory because any portion of the metal fastener exposed to the hot combustion gases will negate the advantages of the composite panels. Composite fasteners may be used instead, however composite fasteners are considerably weakened by the presence of threads and therefore threaded composite fasteners may not reliably secure the panel to its support. Accordingly, it is desirable that a composite fastener assembly either avoid the use of threads, or employ threads in a way that safeguards the integrity of the fastener.
U.S. Pat. Nos. 6,042,315 and 6,045,310 present a unique solution to the problem of attaching a composite panel 30 to a noncomposite back structure 32. The disclosed fastener assembly includes a composite stud 40, a metallic, externally threaded sleeve 90, a metallic pin 65, a spring washer 75, a metal nut 94 and an optional nonmetallic spacer 73. The stud extends through the composite liner 30 and metal back structure 32. The pin 65 secures the threaded sleeve to the tip of the stud with the washer sandwiched between the sleeve and the back structure. The nut completes the assembly. Despite the merits of the disclosed fastener assembly, it suffers the shortcoming of relying on numerous small parts. Even under ideal conditions, assembly or disassembly of these parts can be quite arduous. In addition, installation or removal of the fastener assembly imparts torque to the stud. Torque application is undesirable because a composite stud, despite having good tensile strength, is susceptible to potentially damaging interlaminar shear stresses when exposed to torsion.
Ideally, a fastener assembly for a composite panel will also possess other desirable attributes. For example, when the panel and support are elements of a turbine engine component, no portion of the fastener assembly should project beyond the surface of the panel and into the engine gaspath. This is because the projecting portion of the fastener could introduce undesirable turbulence into the combustion gases or could be easily damaged by any debris or foreign objects present in the gases. In addition, there may be occasions when it is advantageous to avoid the use of threaded elements.
What is needed is a fastener assembly for securing a composite panel to a noncomposite support, and in particular a fastener assembly that relies on few parts, is isolated from torsion during assembly and disassembly, and is flush with the surface of the panel. In some applications, avoidance of threaded elements may also be advantageous.
It is, therefore, an object of the invention to secure a composite panel to a noncomposite support with a simple fastener assembly flush with the panel and isolated from torque application during assembly and disassembly. It is a further object to provide a threadless fastener assembly for those applications where a threaded fastener is undesirable.
According to the invention, a fastener for securing a composite panel to a noncomposite support includes a stud with a notched shank and a collar with a circumferentially extending rib. When assembled, the rib and the notch engage each other to resist axial translation between the stud and the collar. In one embodiment, a spring urges the rib into contact with the notch to tension the stud and clamp the panel to its support. In a second embodiment a nut engages an external thread on the collar to tension the stud and clamp the panel to its support. Either embodiment can be assembled and disassembled without imparting torque to the stud.
The principal advantages of the inventive fastener assembly are its simplicity and its ability to be assembled or disassembled without imparting torque to the stud. Another advantage is the option to secure a panel to a support with a completely a threadless fastener assembly.
These and other features and advantages will become more apparent from the following description of the best mode for carrying out the invention and the accompanying drawings.